The Justice Department gave the green light for Google’s $12.5 billion acquisition of Motorola Mobility on Monday, and also paved the way for Apple, Microsoft and BlackBerry-maker RIM to purchase a trove of mobile phone patents.

In an unusual move, DOJ closed its investigations into all three deals in the hotly competitive smartphone market, seeking to bridge a peace between top tech firms that are snatching up patents at dizzying speed and sparring with rivals in court over their application. At the same time, the agency issued stern warning to tech companies that it would take action against entities that try to wield their patent arsenals to hurt their business rivals.

“The division concluded that the specific transactions at issue are not likely to significantly change existing market dynamics,” according to statement by the Justice Department.

In its decision to clear the transactions, the department’s antitrust division said that the companies’ public commitments including statements sent last week pledging to license standards patents underlying most mobile technology these days lessened their concerns about competition in the market.

“In its analysis of the transactions, the division took into account the fact that during the pendency of these investigations, Apple, Google and Microsoft each made public statements explaining their respective (standard essential patent) licensing practices. Both Apple and Microsoft made clear that they will not seek to prevent or exclude rivals’ products from the market in exercising their SEP rights,” the department said.

However, while praising Apple and Microsoft, the DOJ was hesitant about Google’s pledge. “Google's commitments were more ambiguous and do not provide the same direct confirmation of its SEP licensing policies,” the statement continued.

The three transactions cleared Monday by the DOJ include not only the Google-Motorola Mobility deal, but the $4.5 billion purchase of bankrupt telecom Nortel’s trove of 6,000 mobile patents by a consortium of companies including Microsoft, Apple and RIM — known as Rockstar Bidco.

Apple’s proposal to acquire patents held by CPTN Holdings LLC, formerly owned by Novell, following CPTN's acquisition in April 2011 of those patents on behalf of Apple, Oracle Corporation and EMC Corporation, was also cleared by DOJ on Monday.

The DOJ kept the door open for further review of how the companies are licensing patents, particularly the standards essential patents they developed in conjunction with standards organizations and other companies, which include such widely-used technologies as Wi-Fi and online video.

“In light of the importance of this industry to consumers and the complex issues raised by the intersection of the intellectual property rights and antitrust law at issue here, as well as uncertainty as to the exercise of the acquired rights, the division continues to monitor the use of SEPs in the wireless device industry, particularly in the smartphone and computer tablet markets," the DOJ said in its statement. "The division will not hesitate to take appropriate enforcement action to stop any anticompetitive use of SEP rights.”

The DOJ's move came only hours after regulators from the European Union cleared Google's purchase of Motorola Mobility. The deal still needs the OK from China before it can be sealed.

Some rivals had argued that Google might misuse some of Motorola's standards essential patents, but were pleased at the commitment for an ongoing review. “We are encouraged that the U.S. Department of Justice and the European Commission both have raised concerns about the misuse of standard-essential patents and Google’s failure to address this issue in a satisfactory way. Google’s letter last week only intensified these concerns.

This underscores the importance of further regulatory scrutiny," said Brad Smith, executive vice president and general counsel for Microsoft.

Tony Romm contributed to this report.

This article first appeared on POLITICO Pro at 6:00 p.m. on February 13, 2012.